The present invention is directed to aqueous ink compositions. More particularly, the present invention is directed to ink compositions suitable for use in ink jet printing processes. One embodiment of the present invention is directed to a process for preparing an ink composition which comprises admixing water, a colorant, a base, and phosphorous acid, wherein the pH of the ink is adjusted by a process selected from the group consisting of (a) adding phosphorous acid to an aqueous solution containing the base, and (b) adding the base to an aqueous solution containing phosphorous acid. Another embodiment of the present invention is directed to a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and a base; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding phosphorous acid thereto. Yet another embodiment of the present invention is directed to a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and phosphorous acid; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding a base thereto. The present invention also includes ink compositions prepared by these processes. One embodiment of the present invention is directed to an ink composition which comprises water, a colorant, and phosphorous acid. Another embodiment of the present invention is directed to an ink composition which comprises water, a colorant, and a phosphite salt.
Ink compositions for ink jet printing are known. For example, U.S. Pat. No. 5,062,892 (Halko) discloses ink jet ink compositions containing oxo anions (phosphates, polyphosphates, phosphate esters, arsenate, molybdate, sulfate, sulfite, and oxalate) in an amount of from 9 milligrams per liter to 14 weight percent. The oxo anion additive is included in the ink to reduce kogation in thermal ink jet printers.
In addition, U.S. Pat. No. 4,853,037 (Johnson et al.) discloses an ink composition for printing on plain paper using a thermal ink jet printer. The ink composition comprises at least one member selected from the group consisting of ethylene glycol and diethylene glycol, present in an amount ranging from about 5 to 10 weight percent, dye, ranging from about 1 to 4 weight percent, and the balance water. A biocide ranging from about 0.01 to 0.3 weight percent and/or a buffering agent, such as sodium borate, sodium hydrogen phosphate, or sodium dihydrogen phosphate, ranging in concentration from about 0.05 to 0.5 weight percent may also be included.
U.S. Pat. No. 4,920,361 (Arahara et al.) discloses an image recording method and an image recording apparatus using an ink which is substantially non-adhesive but can be imparted with an adhesiveness when subjected to a pH change. In the image recording method, the ink is subjected to a pattern of pH change to be provided with an adhesive pattern, which is then transferred to a recording medium, such as plain paper, directly or by the medium of an intermediate transfer medium to form an ink pattern corresponding to the pH change pattern.
U.S. Pat. No. 4,838,940 (Kan et al.) discloses an ink which can be imparted with an adhesiveness on application of an electric current. The ink is obtained by impregnating a crosslinked substance such as guar gum or polyvinyl alcohol compound with a liquid dispersion medium such as water. The ink is supplied with a pattern of energy to be provided with an adhesive pattern, which is then transferred to a recording medium, such as plain paper, directly or by the medium of an intermediate transfer medium to form an ink pattern corresponding to the energy pattern applied. The ink further comprises a buffer action-imparting substance and is excellent in storage stability and stability of performances during a continuous use.
Japanese Patent Publication 53-85626 discloses a water-based ink composition for ball point pens which comprises an aqueous water-soluble dye solution and a pH buffer. The pH buffer is either (i) NaOH and KH.sub.2 PO.sub.4, (ii) KH.sub.2 PO.sub.4 and Na.sub.2 HPO.sub.4, (iii) citric acid and Na.sub.2 HPO.sub.4, (iv) KH.sub.2 PO.sub.4 and borax, (v) borax, or (vi) sodium tripolyphosphate, each providing an initial pH of 7 to 9.
Japanese Patent Publication 57-36170 discloses a water-based ink for ink jet printing containing water soluble dyes, a phosphoric ester type surfactant as a humectant, and water.
Copending application U.S. Ser. No. 07/992,240, entitled "ink Compositions and Preparation Processes Thereof," with the named inventor Richard L. Colt, filed Dec. 17, 1992, the disclosure of which is totally incorporated herein by reference, discloses a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and a base; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding polyphosphoric acid thereto. Also disclosed is a process for preparing an ink composition which comprises (a) forming a mixture by admixing water and polyphosphoric acid; (b) adding a colorant to the mixture; and (c) adjusting the pH of the mixture by adding a base thereto. Further disclosed are ink compositions prepared by these processes. Additionally disclosed are processes which comprises incorporating these inks into an ink jet printing apparatus and causing droplets of the ink to be ejected in an imagewise pattern onto a substrate. The presence of polyphosphoric acid and the base create a buffer system in the ink which adjusts the ink to the desired pH, and also enables improved latency and waterfastness.
Although known inks are suitable for their intended purposes, a need remains for improved ink compositions suitable for use in ink jet printing processes. In addition, there is a need for ink compositions which exhibit improved latency and recoverability in ink jet printers. Further, there is a need for ink compositions which exhibit improved latency and which also exhibit acceptable waterfastness, pH, viscosity, and surface tension and generate high quality images. Additionally, there is a need for ink compositions with good latency and acceptable pH for which a wide range of solvents is available so that the ink composition can be tailored for compatibility with specific colorants, improved waterfastness, adjusted viscosity, adjusted surface tension, or other important ink characteristics. There is also a need for ink compositions with both improved latency and improved apparent dry times and/or penetration rates into substrates. Further, there is a need for processes for adjusting the pH of aqueous ink compositions containing dyes wherein no crystallization of salts is observed in the pH-adjusted ink. In addition, there is a need for ink compositions for ink jet printing which are stable at pH values of less than about 7.0.